1. Field of Invention
The invention relates to a discharge lamp of the short arc type which is used, for example, as the light source of a projector, and which is filled with xenon. The invention also relates to a discharge lamp of the short arc type which is used as a light source for semiconductor exposure and which is filled with mercury.
2. Description of Related Art
In a discharge lamp of the short arc type there is normally a trigger wire on the outside surface of the arc tube for reducing the breakdown voltage.
Japanese patent disclosure document JP SHO 51-78586 A discloses as a process for attachment of the trigger wire in a lamp a technique in which hermetically sealing parts which border the two ends of the arc tube are wound once with a trigger wire and in which the ends of the wound trigger wire are twisted with a tool and attached.
Furthermore, Japanese patent disclosure document JP 2002-367566 A (U.S. Pat. No. 6,657,383 B2) discloses a technique in which the following is done:                A metal wire with a resilient property of stainless steel or the like is processed according to the shape of the arc tube in the form of an arc.        Its two ends are formed into spring-like parts which correspond to the diameter of the hermetically sealing parts which extend from both ends of the arc tube.        The spring-like parts are spread, the hermetically sealing parts are bridged with them and they are held by the inherent spring force of the spring-like parts.        Thus, the trigger wire is slipped onto the arc tube and the hermetically sealing parts.        
In the technique shown in Japanese patent disclosure document JP SHO 51-78586 A, the ends of the trigger wire are twisted with a tool and slipped on. Therefore, there is the danger that the twisting tool will come into contact with the glass surfaces of the hermetically sealing parts and the arc tube and will damage the glass surfaces. In a discharge lamp of the short arc type in which the inside of the arc tube during operation of the lamp reaches a high pressure, this damage causes the lamp to break. Since the twisting process is repeated several times, there is also the danger that the trigger wire will rub off the heat insulating film which is formed on the surfaces of the hermetically sealing parts and the arc tube, that the heat insulating film will come off and that the appearance of the lamp will be damaged.
In the trigger wire described in Japanese patent disclosure document JP 2002-367566 A (U.S. Pat. No. 6,657,383 B2), the time for installation of the trigger wire is shortened, by which workability is improved. However, if the duration of operation is lengthened, the trigger wire is exposed to a high temperature during this time. There are the disadvantages here that the resilience is lost and that the spring-like parts, as a result of the loosened shape, fall off of the hermetically sealing parts. When the trigger wire falls off the lamp, the function of the trigger wire which actually supports starting is lost.
As a countermeasure against loosening of the trigger wire of stainless steel as a result of a high temperature, it can be imagined that the wire diameter is increased. However, increasing the wire diameter causes the trigger wire to become hard. The work of installing the trigger wire is made difficult thereby and the actual purpose of shaping it beforehand into a resilient shape to improve workability is lost. The trigger wire, when hardened, furthermore, damages the glass surfaces of the hermetically sealing parts when the hermetically sealing parts are inserted into the elastic parts; this causes the lamp to break. A thick trigger wire in a lamp for semiconductor exposure also shields the light radiated from the lamp; this also hinders the uniformity of the illuminance of the irradiated surface.
As a countermeasure against loosening of the trigger wire of stainless steel as a result of a high temperature, it can be imagined that the wire material is changed. However, since the trigger wire, which is adjacent to the arc tube which reaches 500° C. to 750° C. during lamp operation in the atmosphere, is exposed to a high temperature, the selection of metal wires which have resistance over a long time at a high temperature and which are suited as a spring material is more and more limited. For example, platinum or the like is resistant to a high temperature and is therefore advantageous as an spring material. However, that platinum is expensive and easily burns through are disadvantages, and there are similar disadvantages.